Electroplated Pillars 1 Template Procedure

Figure 2.1 and 2.2 shows a schematic illustration of the fabrication process for the creation of compressive and tensile mechanical testing specimens. In this work, Cu nanopillars were intentionally chosen; however the technique is not limited to these particular metals and can be applied to a wide variety of electroplatable systems. Nanopillar arrays were fabricated on Si substrates ranging in size from ∼1 cm2 _{chips to 100 mm diameter wafers. Prior to applying the PMMA resist, a 20 nm thick Ti}

adhesion layer and a 100 nm thick Au seed layer were deposited on the substrate by electron beam evaporation. This conductive seed layer acts as a cathode in the subsequent electroplating steps. The choice of metal and the thickness of this conductive layer are noncritical, but need to be appropriate for the electrochemical processing, that is, the film does not form an oxide, and for the nanomechanical testing, that is, strong adhesion between the film and the Si substrate. The substrates were spin coated with various dilutions of 950 kD PMMA in anisole (Micro- Chem Corp.). Details of the PMMA resist conditions can be found in Ref. [68]. Generally, pillars meant for compression testing are required to have an aspect ratio of at least 3-to-1 (height-to-diameter) and no greater than 6- to-1. This ensures that nano- pillars are tall enough to experience homogeneous deformation without significant effects of top and bottom constraints, but not so tall as to buckle during compression testing. An aspect ratio of ∼4-to1 was selected as the standard for the nanopillars fabricated in this report. Even though the nanopillar aspect ratio is ultimately governed by plating time, it is imperative that the PMMA resist thickness closely matches the intended nanopillar height. This requirement is critical for the fabrication of taper-free pillars with sub-100 nm diameters and also eliminates the use of excessive electron doses. The resist dilutions were selected such that the spin conditions were maintained between 1500 and 4500 rpm, thus ensuring uniform PMMA films. Following spin coating, the PMMA layer was baked at 180 °C for 15 min. The resist was then exposed using a Leica EBPG5000+ electron beam lithography system operating at an acceleration voltage of 100 kV. For all exposures, the beam current was maintained between 650 and 800 pA and the beam step size was 5 nm. The resolution of electron beam lithography is primarily a function of the electron dosage, whose optimal value depends on the resist type and thickness, minimum feature size, and pattern density. Since these relations are inherently nonlinear, a dose matrix was routinely used in order to empirically determine the optimal exposure conditions. Exposure patterns were computer generated and are extremely flexible, allowing for precise isolation of nanopillars and simultaneous fabrication of the

indicator markers, as the individual nano- pillars were routinely spaced up to 50 µm apart. Immediately following exposure, the PMMA was developed for 60 s in a 1:3 solution of methylisobutylketone (MIBK) and isopropyl alcohol (IPA) followed by a 5 s rinse in IPA.

Figure 2.1 Schematic representation of the FIB-less fabrication steps for compression and tension mechanical testing specimens, adapted from Ref. [68]

Figure 2.2: Schematic of electroplating setup. Image courtesy of D. Jang

2.1.1.2 Electroplating Procedure 2.1.1.2.1 Copper

Adapted from Burek and Greer, Nano Letters 2011 [68]

Following development of the PMMA, the resist template was ready for metal electroplating. Electroplating was performed using a two-electrode configuration in a 1.0 L glass beaker. The Au seed layer underneath the resist template acted as the cathode, and a Pt-coated Nb mesh was used as an insoluble anode. The Cu plating solution was made in house using reagent grade 125 g/L Cu(SO4)·5H2O and a supporting electrolyte of 50 g/L H2SO4. The homemade Cu plating solution was

mixed with deionized water and reagent grade chemicals. The bath temperature was maintained at room temperature for Cu deposition. The plating solution was mechanically stirred and electroplating was performed under both galvanostatic (DC) and reverse pulse (AC) conditions. In DC plating, the

current density for single crystalline pillars with diameters below 250nm was 10 mA/cm2 for Cu electroplating. For AC plating, the cathodic/anodic current was 10 mA/cm2 / 3.5mA/cm2 respectively, and the cathodic pulse was held for 5 s followed by the anodic pulse for 1 s. The electroplating rate was estimated using Faraday’s law, and deposition was stopped when the desired pillar height was achieved. Occasionally, fresh electrolytes are used, but in most cases the solutions were preconditioned electrolytes reused from earlier experiments. Following metal deposition, the PMMA resist was stripped in a bath of acetone at room temperature and rinsed in acetone and IPA. In the case of pillars to be used for tension tests, metal was intentionally overplated for a brief period to form a cap on the top of the pillar. Following removal of the resist template, these caps remain and can be accessed by a set of microgrips in order to tense the pillar

2.1.1.2.2 Iron

Similar procedures as used to produce copper nanopillars have been also used to produce single crystalline iron nanopillars. The same electron-beam lithography templates used to deposit copper were also used to deposit iron. Here the bath was made of 100g/L of iron (II) sulfate heptahydrate (Mallinckrodt Chemicals). The plating conditions were an AC square wave of 15mA/cm2 _forward

current density for 3 seconds and 0mA/cm2 _{reverse current density for 1 second. The zero current}

density intervals are required to minimize the evolution of hydrogen bubbles at the template interface. The voltage for successful plating experiments was found to be between 1.4V and 1.6V. Sometimes, the observed voltage would rapidly increase to greater than 2V. Under these high voltage conditions, the plating rate increases dramatically, and the resulting pillars contain multiple grains. The high voltage is found to coincide with copper plated alligator clips that contain gaps in the copper plating resulting in rapid bath deterioration. In general, the iron bath is relatively unstable, and the bath quality can deteriorate quickly and without warning. This bath deterioration is observed through the evolution of an orange precipitate, likely an iron oxide, and operating voltages at the above electroplating conditions above 2V. As a result, all electroplating using this iron bath is done within a small time window to preserve consistency in the plating results.

Beyond examples of copper and iron single crystals, this templated electroplating approach has been used to produce pillars of several different materials (gold, copper, nickel, tin, and iron) and microstructures (single crystal, nanocrystalline, nano-twinned). For example, multiple different copper microstructures can be produced through control of the electroplating conditions: higher current

densities promotes the growth of nanocrystalline pillars, while very high pulses separated by long periods of zero current can promote the formation of dense nanotwinned pillars.